This invention relates to an apparatus and method for making barrier-coated polyesters, preferably barrier coated polyethylene terephthalate (PET) and articles made therefrom. Preferably the barrier-coated PET takes the form of preforms having at least one layer of a barrier material and the bottles blow-molded therefrom.
The use of plastic containers as a replacement for glass or metal containers in the packaging of beverages has become increasingly popular. The advantages of plastic packaging include lighter weight, decreased breakage as compared to glass, and potentially lower costs. The most common plastic used in making beverage containers today is PET. Virgin PET has been approved by the FDA for use in contact with foodstuffs. Containers made of PET are transparent, thin-walled, lightweight, and have the ability to maintain their shape by withstanding the force exerted on the walls of the container by pressurized contents, such as carbonated beverages. PET resins are also fairly inexpensive and easy to process.
Despite these advantages and its widespread use, there is a serious downside to the use of PET in thin-walled beverage containers: permeability to gases such as carbon dioxide and oxygen. These problems are of particular importance when the bottle is small. In a small bottle, the ratio of surface area to volume is large which allows for a large surface for the gas contained within to diffuse through the walls of the bottle. The permeability of PET bottles results in soft drinks that go “flat” due to the egress of carbon dioxide, as well as beverages that have their flavor spoiled due to the ingress of oxygen. Because of these problems, PET bottles are not suitable for all uses desired by industry, and for many of the existing uses, the shelf-life of liquids packaged in PET bottles is shorter than desired.
U.S. Pat. No. 5,464,106 to Slat, et al, describes bottles formed from the blow molding of preforms having a barrier layer. The barrier materials disclosed are polyethylene naphthalate, saran, ethylene vinyl alcohol copolymers or acrylonitrile copolymers. In Slat's technique, the barrier material and the material to form the inner wall of the preform are coextruded in the shape of a tube. This tube is then cut into lengths corresponding to the length of the preform, and is then placed inside a mold wherein the outer layer of the preform is injected over the tube to form the finished preform. The preform may then be blow-molded to form a bottle. The drawbacks of this method are that most of the barrier materials disclosed do not adhere well to PET, and that the process itself is rather cumbersome.
A family of materials with good barrier characteristics are those disclosed in U.S. Pat. No. 4,578,295 to Jabarin. Such barrier materials include copolymers of terephthalic acid and isophthalic acid with ethylene glycol and at least one diol. This type of material is commercially available as B-010 from Mitsui Petrochemical Ind. Ltd. (Japan). These barrier materials are miscible with polyethylene terephthalate and form blends of 80-90% PET and 10-20% of the copolyester from which barrier containers are formed. The containers made from these blends are about 20-40% better gas barriers to CO2 transmission than PET alone. Although some have claimed that this polyester adheres to PET without delamination, the only preforms or containers disclosed were made with blends of these materials.
Another group of materials, the polyamine-polyepoxides, have been proposed for use as a gas-barrier coating. These materials can be used to form a barrier coating on polypropylene or surface-treated PET, as described in U.S. Pat. No. 5,489,455 to Nugent, Jr. et al. These materials commonly come as a solvent or aqueous based thermosetting composition and are generally spray coated onto a container and then heat-cured to form the finished barrier coating. Being thermosets, these materials are not conducive to use as preform coatings, because once the coating has been cured, it can no longer be softened by heating and thus cannot be blow molded, as opposed to thermoplastic materials which can be softened at any time after application.
Another type of barrier-coating, that disclosed in U.S. Pat. No. 5,472,753 to Farha, relies upon the use of a copolyester to effect adherence between PET and the barrier material. Farha describes two types of laminates, a three-ply and a two-ply. In the three-ply laminate, an amorphous, thermoplastic copolyester is placed between the barrier layer of phenoxy-type thermoplastic and the layer of PET to serve as a tie layer to bind the inner and outer layers. In the two-ply laminate, the phenoxy-type thermoplastic is first blended with the amorphous, thermoplastic copolyester and this blend is then applied to the PET to form a barrier. These laminates are made either by extrusion or by injection molding wherein each layer is allowed to cool before the other layer of material is injected.
PCT Application No. PCT/US95/17011, to Collette et al., which was published on Jul. 4, 1996, describes a method of cooling multilayer preforms. The disclosed apparatus comprises a rotary turret having multiple faces, each face carrying an array of cores. The cores are inserted into corresponding mold cavities. Multiple melt streams are brought together and coinjected into each cavity to form a multilayer preform on each core. After the preform is injected, the cores are removed from the cavities and the turret is rotated, presenting a new set of cores to the mold cavities. The just-injected cavities remain on the cores cooling while preforms are formed on other arrays of cores. The drawbacks of the Collette application include that coinjection results in preforms that are inconsistent and have unpredictable layering. Thus, distribution of barrier materials in such a preform would be unpredictable and would result in a preform having unreliable barrier properties.
Since PET containers can be manufactured by injection molding using only a single injection of PET, manufacture is relatively easy and production cycle time is low. Thus, PET containers are inexpensive. Even if known barrier materials can be bonded to PET to create a saleable container with reliable barrier properties, methods and apparatus for making such containers within a competitive cycle time and cost have not been devised. Production cycle time is especially important because a lower cycle time enables a manufacturer to make more efficient use of its capital equipment. Thus, low cycle time enables higher volume and less expensive production of containers. Cost-effective production would be necessary to develop a viable alternative to monolayer PET containers.
Thus, the need exists for an apparatus and method for making barrier-coated PET preforms and containers which are economical, cosmetically appealing, easy to produce, and have good barrier and physical properties remains unfulfilled.